With the current growing interest in environmental issues, a lot of research has been conducted on electric vehicles and hybrid electric vehicles that may replace vehicles, such as gasoline vehicles, diesel vehicles, and the like, using fossil fuels, which has been pointed out as one of main causes of atmospheric pollution. Although a nickel metal hydride secondary battery has been mainly used as a power source for such electric vehicles, hybrid electric vehicles, and the like, research on the use of lithium secondary batteries having a high energy density and discharge voltage, a long cycle lifespan and a low self-discharge rate has been actively conducted, and some of the lithium secondary batteries are on a stage of commercialization.
A carbon material has been mainly used as a negative electrode active material for such lithium secondary batteries, and the use of lithium metals, sulfur compounds, and the like is under consideration. Also, lithium-containing cobalt oxides (LiCoO2) have been mainly used as positive electrode active materials, and the use of lithium-containing manganese oxides such as LiMnO2 having a lamellar crystal structure, LiMn2O4 having a spinel crystal structure, and the like, and lithium-containing nickel oxides (LiNiO2) is under consideration. Further, various materials such as liquid electrolytes, solid electrolytes, polymer electrolytes, and the like have been used as electrolytes.
The lithium secondary batteries have drawbacks in that an internal short-circuit phenomenon may occur due to low stability to heat at a high temperature (e.g., 90° C. or higher), and the batteries swell up and explode. Particularly, when a liquid electrolyte is used, leakage of the electrolyte may occur. For this reason, a solid electrolyte or a polymer electrolyte was presented as an alternative electrolyte, but no satisfactory lithium ion conductivity may be secured.
In recent years, compounds having a chemical structure such as Li3OBr or Li3OCl, that is, a lithium-rich antiperovskite (hereinafter referred to as ‘LiRAP’) crystal structure, have been suggested. Because such compounds have very excellent lithium ion conductivity and are also stable at high temperature, the compounds have been researched as an alternative to next-generation electrolytes.
The antiperovskite crystal structure refers to a structure similar to perovskite, that is, a structure in which there are different positions between cations and other constituent elements in the crystal structure. The perovskite structure is generally represented by ABX3, wherein A represents a monovalent cation, B represents a bivalent cation, and X represents a monovalent anion. In the antiperovskite structure (ABX3), X refers to a cation such as an alkali metal, and A and B refer to an anion. Hundreds of different types of the perovskite and antiperovskite crystal structures are known depending on which atoms (or functional groups) exist on A, B and X, and also have different electrical characteristics in conductors, semiconductors, and non-conductors.
Xujie Lu et al. suggested Li3OCl as LiRAP, wherein the material has a high level of ion conductivity of 0.85×10−3 S/cm at room temperature, and suggested that the material may be used as an electrolyte because the material has excellent stability at a high temperature due to its orthorhombic crystal structure having a tetragonal phase [Yusheng Zhao et al., Superionic Conductivity in Lithium-Rich Anti-Perovskites, J. Am. Chem. Soc., 2012, 134 (36), pp 15042-15047; US Patent Application No. 2013-0202971].
In the crystal structure such as LiRAP, a dopant is intentionally added as a foreign substance to enhance ion conductivity. In this regard, Yusheng Zhao et al. suggested that Br-doped Li3O (Cl0.5,Br0.5) has an ion conductivity of 10−2 S/cm, the value of which may be higher than that of Li3OCl (10−3 S/cm), and thus may be used as a solid electrolyte [Yusheng Zhao et al., Superionic Conductivity in Lithium-Rich Anti-Perovskites, J. Am. Chem. Soc., 2012, 134 (36), pp 15042-15047].
Similarly, Yusheng Zhao and M. H. Braga et al. suggested compounds having a (Ba, Li)3OCl structure in which lithium ions are substituted with other metal ions, disclosing that the material may be used in batteries which need to be operated at a high temperature, such as metal-air batteries or all-solid-state batteries, because the material may enhance ion conductivity at room temperature and exhibits non-flammable characteristics at a high temperature.